1. Field of the Invention
The present invention relates to spin dependent conduction devices, and more particularly, spin dependent conduction devices comprising a plurality of ferromagnetic tunnel junctions and utilizing a discrete energy level of ferromagnetic layer between the ferromagnetic tunnel junctions.
2. Discussion of the Background
A magnetoresistance effect (MR) is a phenomena in which the resistance of ferromagnetic material, such as NiFe alloy, varies dependently on the intensity of applied magnetic fields. An MR element utilizes such phenomena and is used as a magnetic sensor or a magnetic head. The MR amplitude of the NiFe alloy is around 2 to 3%; however, a larger amplitude is required to obtain magnetic recording of higher density.
A metal artificial lattice film which exhibits a giant magnetoresistance effect (GMR) was reported in publications, 61 Phys. Rev. Lett., 2472(1988), 94 J. Mag. Mater., L1(1991), and 66 Phys. Rev. Lett., 2152(1991). The film comprises a plurality of ferromagnetic layers and nonmagnetic layers interposed between each of the ferromagnetic layers. The electrons scattering characteristic in the film depends on spin directions of ferromagnetic layers of the film. The film has about a 10 or 20% MR amplitude. Many layers are needed to obtain a high MR amplitude, and the saturation magnetization is as much as several Tesla (T). These characteristics are not preferable for applying the film to the magnetic head.
Another mechanism has been identified in which the resistance between two uncoupled ferromagnetic layers is observed to vary as the cosine of the angle between the magnetization of the two layers and is independent of the direction of current flow. This is termed as spin valve (SV) magnetoresistance in U.S. Pat. No. 5,206,590. The MR amplitude of the mechanism is about 4-8%, and the specific resistance is tens of micro ohms-cm. Therefore, a large current is required to sense small applied magnetic fields.
It was also reported that a vertical magnetoresistance effect is obtained when current flows in a vertical direction of the film surface of an artificial lattice multilayer in 66 Phys. Rev. Lett., 3060(1991). The resistance of the film is too small because of a short current path and the use of a metal multilayer. Also, in order to obtain the effect at room temperature, it is necessary to form a film having a submicron pattern.
It was reported that the GMR could be obtained by using a granular ferromagnetic film. The film comprises dispersed magnetic fine-grains in a nonmagnetic metal material layer as reported in 68 Phys. Rev. Lett., 3745(1992). Spins of the fine-grains have mutually irregular directions, and the film shows high resistance with no applied field. When the magnetic fields are applied, the resistance of the film decreases. The fine-grains have super-paramagnetism and large saturation magnetization fields.
Another mechanism of the GMR differs from the spin dependent scattering. The mechanism is obtained by a structure including a ferromagnetic layer/ an insulator layer/ a ferromagnetic layer. The coercive force of one ferromagnetic layer is larger than that of the other layer, and a tunnel current is obtained at a specific voltage. Resistance changes depend on whether the spin direction of both ferromagnetic layers are parallel or antiparallel. The spin direction of the small coercive force layer is controlled by applied magnetic fields. The film structure and the mechanism is so simple and it may show about 20% MR amplitude at room temperature. However, the film thickness of the insulator layer is less than several nanometers, and it is difficult to form a stable thin insulator layer. Also, the resistance of several square micrometers area becomes the order of mega ohms, and low speed performance and increasing noise become a problem when a high resistance insulator layer is used (see 74 Phys. Rev. Lett., 3273 (1995)).
Double tunnel junctions of Fe/Ge/Fe/Ge/ferromagnetic material are expected by theoretical calculations that a large MR amplitude due to spin polarization resonance tunnel effect may be shown (see B56 Phys. Rev., 5484 (1997)). However, the MR amplitude was calculated for a temperature of 8.degree. K., and the device was not actually formed at that time.
Other tunnel junctions having Al.sub.2 O.sub.3 /granular layer/Al.sub.2 O.sub.3 were also reported in R56 Phys. Rev., R5747 (1997). The granular layer comprises Co grains formed in an Al.sub.2 O.sub.3 material. Each of the Co grains has a diameter of several nanometers, does not have uni-direction and shows paramagnetic at 120.degree. K. Therefore, the granular layer does not spin switch at low temperature even though it is provided with a large magnetic field of more than 0.5 Tesla and the device does not show spin resonance tunnel effect.
A tri-terminal device such as a spin transistor comprising a ferromagnetic metal layer, a nonmagnetic metal layer, and another ferromagnetic metal layer was also reported. An output voltage is obtained between one of the ferromagnetic metal layers and the nonmagnetic layer when a voltage is applied between those layers, and the positive/negative characteristic of the output voltage depends on whether the spin directions of the two ferromagnetic metal layers are parallel or antiparallel, as reported in 79 J. Appl. Phys., 4724(1996). However, the metal layers of this transistor prohibit an output of more than nanovolts and no gain current is obtained.
A Coulomb Blockade effect was also reported to exhibit the MR in 66 J. Phys. Soc. Jpn., 1261(1997). The term, coulomb blockade, describes the phenomena that energy increases about Ec=e.sup.2 /2C when an electron tunnels at a small capacitor C. At a small capacitor, the increase of Ec prohibits tunneling of the electron. However, a high order tunnel current (resonant tunnel current) flows and the resistance of the device, which is in proportion to the product of the two tunnel junction's resistance, increases. Therefore, the MR amplitude increases.
A Magnetic Random Access Memory (MRAM) is also reported in which one ferromagnetic layer of stacked ferromagnetic layer/nonmagnetic layer/ferromagnetic layer is used for recording and another ferromagnetic layer is used for reproducing layer. The device requires a current source for providing magnetic fields to the device at both recording and reproducing.
Conventional semiconductor devices utilize electric charge of electrons or holes and do not utilize spin of the electrons.
The conventional semiconductor devices and the resonant tunnel device utilized the electric charge of the electron or holes and did not utilize spin of the electron.
The conventional spin dependent conduction devices utilizing the magnetic spin are the spin valve (SV) element, and the ferroelectric tunnel junction device. The MR amplitude of these spin conduction devices is less than 20%. As a result, the reproducing sensitivity and output voltage are small. The MRAM must be provided with a current source for providing magnetic fields.
The conventional spin transistor exhibits small output voltage and has an insufficient current gain.